The present invention relates to imaging devices for detecting a radiation distribution due to positron annihilations in an organ of interest of a living subject.
Although positron imaging enables the use of a whole new class of radiopharmaceuticals of great physiological significance, the development of the appropriate instrumentation has lagged compared, for example, to gamma imaging. Positron imaging devices can be grouped into two categories: (a) those using a multiplicity of discrete detector units, and a similar multiplicity of coincident circuits to accommodate these detectors; and (b) those using at least one Anger-type scintillation camera having a single crystal, together with a second detector, and a single coincidence circuit. An early report by H. O. Anger printed for the U.S. Atomic Energy Commission entitled "Scintillation in Positron Cameras" (UCRL-8640, Aug. 12, 1959) applies the underlying concept of the original Anger scintillation camera (see U.S. Pat. No. 3,011,057) with some appropriate modifications and alterations to detect positron events.
Regardless of the category of system, the goals are identical; that is, to achieve high sensitivity and resolution, and at the same time high count rate capability. Although the camera-based systems are generally superior in resolution, heretofore they have lacked count rate capability comparable to the discrete detector systems (see for example H. O. Anger, "Radioisotope Cameras: Instrumentation in Nuclear Medicine", Vol. 1, G. J. Hine, Editor, New York, Academic Press, 1967; G. L. Brownell and C. A. Burham, "Recent Developments in Positron Scintigraphy: Instrumentation in Nuclear Medicine", Vol. 2, G. J. Hine and J. A. Sorenson, Editors, New York, Academic Press, 1973; and P. J. Kenny, "Spatial Resolution and Count Rate Capacity of a Positron Camera: Some Experimental and Theoretical Considerations", International Journal of Applied Radiation and Isotopes, Vol. 22, Permagon Press, Pages 21-28, 1971). The results of the above investigations have lead to the conclusion that the useful count rate capability of a positron camera is only a small fraction of the actual count rate.
Accordingly, the interest by other workers has been biased toward development of discrete detector positron devices. Recently, the first commercial availability of positron devices has been of the discrete detector type: a device developed by Dr. G. Brownell is offered by Cyclotron Corporation, and a system developed by Drs. M. Phelps and M. Ter Pogossian is offered by Ortec, Inc. Although both devices have good count rate capability, they are limited in sensitivity, resulting in long imaging times with many tagged pharmaceuticals of interest.
Meanwhile, efforts have continued at development of an improved Anger camera-based positron imager. One recent development of importance has been the invention of graded radiation absorbers with plural coincidence channels for the Anger detectors, to enable both the primary radiation as well as the Compton events, which formerly would have been lost, to be processed, thereby improving the overall count rate. See U.S. Pat. No. 3,955,088, the disclosure of which is incorporated herein by reference.
The advent of tomographic imaging in other imaging modalities, especially X-ray, has made the achievement of satisfactory count rates of still greater importance. Indeed, transverse tomographic imaging has not heretofore been fully successful for Anger camera positron devices because of the sensitivity and count rate problem and because of the need for better data processing. It has been attempted, for example, to implement a rotational transverse mode by adding collimators consisting of parallel slits to a pair of opposed camera detectors, rotating, then utilizing the same algorithms and reconstruction techniques as in X-ray computerized tomography. But it has been found that the collimators reduce sensitivity and counting statistics too drastically for such a system to be of any great general utility.
The above-mentioned discrete detector-based imagers have also been used in attempts to provide a rotational transverse mode, but the devices so far produced have been of limited capability due to the need for irises or other collimation, the possibility of only a limited number of resolution points over the detector arrays, the lack of continuity between image slices, and the capability for only one or at best a few image slices simultaneously per scan.